1. Field of the Invention
The present invention relates to a rubber composition for a tire tread. More specifically it relates to a rubber composition for a tire tread having a high frictional force on ice and a pneumatic tire using the same as a tread portion thereof.
2. Description of the Related Art
Various proposals have been made heretobefore to formulate a hard foreign substance or a hollow particle into a rubber to thereby form a micro irregularity on the surface of a rubber layer, whereby a water-film present on the surface of the ice is removed during a running of a tire and the friction on the ice is increased, but practically satisfactory results have not been obtained yet. Examples of such proposals are disclosed in, for example, JP-A(Kokai)-60-258235 (i.e., micropowder of ceramic), JP-A-2-274740 (i.e., finely divided plant), and JP-A-281052 (i.e., fine metal particle). However, according to these proposals, there are caused other problems such as the increase in the hardness of the rubber and the loss of flexibility of the rubber whereby the resultant tire is difficult to follow the contours of the road. Typical examples of formulating hollow particles are disclosed in JP-A-2-170840, JP-A-2-208336 and JP-A-4-5543. However, these proposals have problems that the hardness of the rubber is also increased and that the hollow particles are broken during the mixing thereof.